Beirut — The death toll from coordinated attacks by Islamic State fighters on a usually peaceful southern city and surrounding countryside has climbed to 216, a local health official said Thursday, in the worst violence to hit the area since the country's conflict began.

Mass funerals were held in the city of Sweida on Thursday, a day after the wave of attacks that began in the early hours of the mourning and lasted for hours. The city was the scene of several suicide bombings, including one at a busy vegetable market that left a scene of devastation and set in motion the coordinated assaults.

IS militants also attacked a number of villages in the northeast of the province, also called Sweida, where local militias and residents took up arms to fight the advancing militants.

The Britain-based Syrian Observatory for Human Rights put the death toll at 246, including 111 members of local militias who fought IS militants who swarmed their villages. At least 135 civilians were among the killed, the Observatory said.

Hassan Omar, a government health official in Sweida province, said on Thursday that at least 150 people were wounded in the attacks; some of them were in critical condition.

An activist-operated Facebook page called Sweida News Network said many of the killed were shot in the head. The SNN said the militants sneaked into the villages under the cover of darkness, shooting residents as they slept.

The Observatory also reported bodies found killed inside homes and that the militants had also abducted some residents, their fate unknown.

The rare attacks in Sweida, populated mainly by Syria's minority Druze, came amid a government offensive elsewhere in the country's south. Government forces are battling an IS-linked group near the frontier with Israeli-occupied Golan Heights and near the border with Jordan. The group also has a small presence on the eastern edge of Sweida province, and in the desert in the adjacent Homs province.

Since their offensive in June, Syrian President Bashar Assad's forces have retaken territories controlled by the rebels along the Golan Heights frontier and are now fighting militants in the country's southern tip.

IS has been largely defeated in Syria and Iraq, but still has pockets of territory it controls in eastern and southern Syria.

The extremist group boasted that its "soldiers" killed more than 100 people in Sweida.

In a statement posted on the group's social media channels, it said its militants carried out surprise attacks on government and security centres, sparking clashes with Syrian troops and allied militias.

The Islamic State group posted no death toll for its own men in Wednesday's fighting.